Measuring the temperature of a semiconductor device during various stages of processing is necessary for characterizing and controlling processing of the device. A preferred method of conducting such temperature measurements is by contactless temperature monitoring.
Commonly assigned U.S. Pat. No. 5,022,765, entitled "NULLING OPTICAL BRIDGE FOR CONTACTLESS MEASUREMENT OF CHANGES IN REFLECTIVITY AND/OR TRANSMISSIVITY", issued to Guidotti et al , incorporated herein by reference, discloses an apparatus for conducting contactless temperature monitoring by implementing a hulling optical bridge for measuring the difference in relative power of more than one light beam. As shown in FIG. 1, labelled as "Prior Art", an optical beam source 1 is linearly polarized by a polarizer 2 such that the electric field direction of the emerging beam makes an angle (.PHI.) with respect to an optic axis of a polarization sensitive beamsplitter prism 3. The prism 3 splits the incident light into two orthogonally polarized beams, beam A and beam B. The power of the beams A,B depends on the angle .PHI., and the divergence angle (.THETA.) of the beams A,B depends on the design properties of the prism 3. As .PHI. is varied by rotating polarizer 2, the difference in power between the two beams (A-B) is varied while the total power emerging from the prism 3 remains constant. The polarizer/prism combination is therefore used as a continuously variable beamsplitter. Beam A is directed from the prism 3 to a reflector 4, and the reflected beam A' is then directed to a photodetector 5. The other beam B is directed to the sample to be measured and the reflected beam B' is reflected from the sample surface at near-normal incidence and directed to a photodetector 6. The photo-current from photodetectors 5 or 6 is proportional to the power in beams A' or B', respectively The photo-current from detectors 5 and 6 is fed to a balance circuit 7, where a difference signal (A'-B') is amplified by amplifier 8 and measured.
Power fluctuations in the light source are eliminated by the null-point measurement of the difference signal (A'-B') which converts a measurement of change in reflectivity or transmissivity into a measurement of the angle .PHI.. This is accomplished by rotating the polarizer 2 by an angle so as to maintain the null condition (A'-B') at all times. The null condition renders the device measurement independent of source power fluctuations because whatever the source variation, variations in A and B are identical in magnitude, and occur in synchronism and therefore are always subtracted. The null condition is automatically, continuously maintained during a measurement by a feedback loop 9 which governs the rotation of the polarizer 2 as dictated by the sign and magnitude of the error signal (A'-B').
Thus, if a change in sample temperature causes a change in sample reflectivity, then the reflected power in beam A' will vary, and the difference (A'-B') will deviate from null. The polarizer angle .PHI. is then rotated by an amount necessary to restore null. The amount of rotation is proportional to the change in sample temperature for small angular variations. Unfortunately, the use of such a nulling optical bridge apparatus has been found to be somewhat impractical in many applications. Since the polarizer 2 must be mechanically rotated, a motor and associated mechanical linkage is required to accomplish such rotation. These components have been found to be overly large and cumbersome, and highly expensive, causing difficulty in incorporating the apparatus in a manufacturing environment. Although the components can be downsized to a certain extent, downsized components reduce the accuracy in accomplishing the required small incremental rotations of the polarizer 2. For instance, it has been found that the polarizer 2 must be rotated approximately 1/1000 of a degree to reflect a change of one degree celsius in a sample. Further, mechanically rotating the polarizer 2 for balancing has the limitation of not being capable of adequately compensating for unpolished or slightly polished wafers. In this regard, the degree by which the polarizer 2 must rotate for compensating for the low reflectivity of such wafers is too extreme for the mechanical components to properly achieve. Thus, the reflectivity and/or transmissivity of unpolished or slightly polished wafers cannot be properly measured using this mechanical apparatus.
Thus, there remains a need for a cost-effective nulling optical bridge which eliminates the problems associated with physical rotation of a polarizer for nulling, and which is easily adaptable for use in a manufacturing environment.